This invention relates to an electrophotographic light-sensitive medium prepared using a novel dis-azo compound containing therein at least two azomethine groups.
Various types of light-sensitive media bearing an electrically conductive layer and an organic pigment-containing layer provided on the electrically conductive layer have heretofore been known, including:
(1) a light-sensitive medium as disclosed in Japanese Patent Publication No. 1667/1977 in which a layer prepared by dispersing a pigment in an insulative binder is provided on an electrically conductive layer;
(2) a light-sensitive medium as disclosed in Japanese Patent Application (OPI) Nos. 30328/1972 (corresponding to U.S. Pat. No. 3,894,868) and 18545/1972 (corresponding to U.S. Pat. No. 3,870,516) in which a layer prepared by dispersing a pigment in a charge transport substance or a charge transport medium comprising the charge transport substance and an insulative binder (which may also be a charge transport substance) is provided on an electrically conductive layer;
(3) a light-sensitive medium as disclosed in Japanese Patent Application (OPI) No. 105537/1974 (corresponding to U.S. Pat. No. 3,837,851) which comprises an electrically conductive layer, a charge generation layer containing a pigment, and a charge transport layer; and
(4) a light-sensitive medium as disclosed in Japanese Patent Application (OPI) No. 91648/1974 in which an organic pigment is added to a charge transfer complex.
As pigments for use in these light-sensitive media, a number of pigments, such as phthalocyanine based pigment, polycyclic quinone based pigment, azo based pigment and quinacridone based pigment, have been proposed, but few of them have been put in practice.
The reason for this is that these organic photoconductive pigments are inferior in sensitivity, durability, etc., to inorganic pigments such as Se, CdS, ZnO, etc.
However, light-sensitive media prepared using inorganic photoconductive pigments also suffer from disadvantages.
For example, with a light-sensitive medium prepared using Se, crystallization of Se is accelerated by heat, moisture, dust, finger print, etc., and in particular, when the atmospheric temperature of the light-sensitive medium exceeds about 40.degree. C., the crystallization becomes significant, resulting in a reduction in charging properties and formation of white spots in an image. Although Se-based light-sensitive medium can theoretically produce about 30,000 to 50,000 copies, it often fails to produce so many copies because it is adversely influenced by the environmental conditions of the location where the copying machine in which it is used is placed.
In the case of a CdS-based light-sensitive medium covered with an insulative layer, its durability is nearly equal to that of the Se-based light-sensitive medium. Additionally, use of CdS results in deterioration of the moisture resistance of the CdS-based light-sensitive medium and it is very difficult to improve this poor moisture resistance. At the present time, therefore, it is necessary to provide an auxiliary means, e.g., a heater.
With a ZnO-based light-sensitive medium, the sensitization thereof is caused by the use of dyes exemplified by Rose Bengale and, therefore, problems such as deterioration due to corona charging and discoloration of the dye by light arise. At the present time, it is generally believed that only about 1,000 copies can be produced by the Se-based light-sensitive medium.
Furthermore, the Se-based light-sensitive medium is expensive and causes pollution problems, as is also the case with the CdS-based light-sensitive medium.
The sensitivity of conventional light-sensitive media, when expressed as an exposure amount for half decay (E 1/2), is as follows: a Se-based light-sensitive medium which is not sensitized, about 15 lux.sec; a Se-based light-sensitive medium wich is sensitized, about 4 to 8 lux.sec; a CdS-based light-sensitive medium, about equal to that of the sensitized Se-based light-sensitive medium; and a ZnO-based light-sensitive medium, about 7 to 12 lux.sec.
When the light-sensitive medium is used in a PPC (plane paper copior) copying machine (manufactured by Copyer Co., Ltd.), its sensitivity is desirably less than 20 lux.sec as E 1/2, whereas when used in a PPC copying machine whose rate of duplication is higher, its sensitivity is more preferably 15 lux.sec or less as E 1/2. Of course, those light-sensitive media having lower sensitivities than above described can also be used, depending on the purpose for which they are used, i.e., cases where the light-sensitive medium is not necessary to be repeatedly used, such as, for example, cases where the light-sensitive medium is used as a coating paper and a toner image is directly formed on the coating paper in copying of a drawing, etc.